One of the many challenges in food service is keeping prepared foods at a desired temperature. Hot foods often lose their texture and flavor when they are cooled and must therefore be served immediately upon preparation. This is often very difficult to do, particularly when food needs to be prepared in advance. On the other hand, it is preferred that certain other types of dishes, such as salads, be served cold. If such dishes are left outside of a refrigerator for any length of time, they will likely warm to room temperature, making them less palatable.
Various devices have been developed for keeping food at a desired temperature (hot or cold) after preparation. Heat lamps, for example, have been extensively used in fast food restaurants for keeping hamburgers and sandwiches warm before serving. Electric hot plates have also been used to keep foods warm. There are also buffet serving trays, which keep food warm by placing a heat source, such as a small flame, underneath the tray. The disadvantages of these devices, however, are that they are not easily portable and require either connection to an external power source or a potentially dangerous open flame to supply the heat. Similarly, cold packs and ice are the most common means used to keep cold dishes at a desired temperature. These methods, however, suffer the disadvantage of melting and creating a soggy mess. Moreover, separate devices or means are typically required to keep food either warm or cold.
It is therefore desirable to have a single device that is capable of keeping food either warm or cold. It is also desirable to have such a device that is easily portable and does not require connection to an external power source to supply the heat.